<!DOCTYPE html>
<html lang="en">

<head>
	<meta http-equiv="content-type" content="text/html; charset=utf-8">
	<meta content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" name="viewport">
	
	<!-- title -->
	
	<title>
	
		CMS垃圾收集器 | 
	 
	对线面试官
	</title>
	
	<!-- keywords,description -->
	 
		<meta name="description" content="Java一线大厂面试" />
	

	<!-- favicon -->
	
	<link rel="shortcut icon" href="/luffy/favicon.ico">
	


	<!-- search -->
	<script>
		var searchEngine = "https://www.baidu.com/s?wd=";
		if(typeof searchEngine == "undefined" || searchEngine == null || searchEngine == ""){
			searchEngine = "https://www.google.com/search?q=";
		}
		var homeHost = "";
		if(typeof homeHost == "undefined" || homeHost == null || homeHost == ""){
			homeHost = window.location.host;
		}
	</script>


	
<link rel="stylesheet" href="/luffy/css/main.css">

	
<link rel="stylesheet" href="https://cdn.staticfile.org/font-awesome/4.7.0/css/font-awesome.min.css">

	
<link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/highlightjs/cdn-release@9.17.1/build/styles/darcula.min.css">

	
<link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/fancyapps/fancybox@3.5.7/dist/jquery.fancybox.min.css">


	
<script src="https://cdn.jsdelivr.net/npm/jquery@3.5.1/dist/jquery.min.js"></script>

	
<script src="https://cdn.jsdelivr.net/gh/fancyapps/fancybox@3.5.7/dist/jquery.fancybox.min.js"></script>

	
<script src="https://cdn.jsdelivr.net/gh/highlightjs/cdn-release@9.17.1/build/highlight.min.js"></script>

	
<script src="https://cdn.jsdelivr.net/npm/jquery-pjax@2.0.1/jquery.pjax.min.js"></script>

	
<script src="/luffy/js/main.js"></script>

	
		
<script src="https://cdn.jsdelivr.net/npm/leancloud-storage/dist/av-min.js"></script>

		
<script src="https://cdn.jsdelivr.net/npm/valine@1.3.10/dist/Valine.min.js"></script>

	
	
<meta name="generator" content="Hexo 5.4.0"></head>

<body>
	<script>
var _hmt = _hmt || [];
(function() {
  var hm = document.createElement("script");
  hm.src = "https://hm.baidu.com/hm.js?3efe99c287df5a1d6f0d02d187e403c1";
  var s = document.getElementsByTagName("script")[0]; 
  s.parentNode.insertBefore(hm, s);
})();
</script>

<header id="header">
    <a id="title" target="_blank" rel="noopener" href="https://mp.weixin.qq.com/mp/appmsgalbum?__biz=MzU4NzA3MTc5Mg==&action=getalbum&album_id=1657204970858872832&scene=126#wechat_redirect" class="logo">对线面试官</a>

	<ul id="menu">
		
	

	

		<li class="menu-item">
			<a href="https://gitee.com/zhongfucheng/Java3y" class="menu-item-link" target="_blank">
				【戳这里】获取更多原创干货💪💪
			</a>
		</li>
		<li class="menu-item">
			<a href="https://gitee.com/zhongfucheng/Java3y" class="menu-item-link" target="_blank">
				<i class="fa fa-github fa-2x"></i>
			</a>
		</li>
	</ul>
</header>

	
<div id="sidebar">
	<button id="sidebar-toggle" class="toggle" ><i class="fa fa-arrow-right " aria-hidden="true"></i></button>
	
	<div id="site-toc">
		<input id="search-input" class="search-input" type="search" placeholder="按回车全站搜索">
		<div id="tree">
			

			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										00-面试前准备
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/10/30/00-%E9%9D%A2%E8%AF%95%E5%89%8D%E5%87%86%E5%A4%87/01.%E7%AE%80%E5%8E%86/">
										01.简历
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										01-Java基础
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/01-Java%E5%9F%BA%E7%A1%80/01.%20%E6%B3%A8%E8%A7%A3/">
										01. 注解
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/01-Java%E5%9F%BA%E7%A1%80/02.%20%E6%B3%9B%E5%9E%8B/">
										02. 泛型
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/01-Java%E5%9F%BA%E7%A1%80/03.%20JavaNIO/">
										03. JavaNIO
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/01-Java%E5%9F%BA%E7%A1%80/04.%20%E5%8F%8D%E5%B0%84%E5%92%8C%E5%8A%A8%E6%80%81%E4%BB%A3%E7%90%86/">
										04. 反射和动态代理
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										02-Java并发
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/02-Java%E5%B9%B6%E5%8F%91/01.%20%E5%A4%9A%E7%BA%BF%E7%A8%8B%E5%9F%BA%E7%A1%80/">
										01. 多线程基础
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/02-Java%E5%B9%B6%E5%8F%91/02.%20CAS/">
										02. CAS
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/02-Java%E5%B9%B6%E5%8F%91/03.%20synchronized/">
										03. synchronized
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/02-Java%E5%B9%B6%E5%8F%91/04.%20AQS%E5%92%8CReentrantLock/">
										04. AQS和ReentrantLock
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/02-Java%E5%B9%B6%E5%8F%91/05.%20%E7%BA%BF%E7%A8%8B%E6%B1%A0/">
										05. 线程池
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/02-Java%E5%B9%B6%E5%8F%91/06.%20ThreadLocal/">
										06. ThreadLocal
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/02-Java%E5%B9%B6%E5%8F%91/07.%20CountDownLatch%E5%92%8CCyclicBarrier/">
										07. CountDownLatch和CyclicBarrier
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/02-Java%E5%B9%B6%E5%8F%91/08.%20%E4%B8%BA%E4%BB%80%E4%B9%88%E9%9C%80%E8%A6%81Java%E5%86%85%E5%AD%98%E6%A8%A1%E5%9E%8B/">
										08. 为什么需要Java内存模型
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/02-Java%E5%B9%B6%E5%8F%91/09.%20%E6%B7%B1%E5%85%A5%E6%B5%85%E5%87%BAJava%E5%86%85%E5%AD%98%E6%A8%A1%E5%9E%8B/">
										09. 深入浅出Java内存模型
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										03-Java容器
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/03-Java%E5%AE%B9%E5%99%A8/01.%20List%E9%9B%86%E5%90%88/">
										01. List集合
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/03-Java%E5%AE%B9%E5%99%A8/02.%20Map%E9%9B%86%E5%90%88/">
										02. Map集合
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										04-Java虚拟机
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/04-Java%E8%99%9A%E6%8B%9F%E6%9C%BA/01.%20Java%E7%BC%96%E8%AF%91%E5%88%B0%E6%89%A7%E8%A1%8C%E7%9A%84%E8%BF%87%E7%A8%8B/">
										01. Java编译到执行的过程
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/04-Java%E8%99%9A%E6%8B%9F%E6%9C%BA/02.%20%E5%8F%8C%E4%BA%B2%E5%A7%94%E6%B4%BE%E6%9C%BA%E5%88%B6/">
										02. 双亲委派机制
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/04-Java%E8%99%9A%E6%8B%9F%E6%9C%BA/03.%20JVM%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84/">
										03. JVM内存结构
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/04-Java%E8%99%9A%E6%8B%9F%E6%9C%BA/04.%20%E5%9E%83%E5%9C%BE%E5%9B%9E%E6%94%B6%E6%9C%BA%E5%88%B6/">
										04. 垃圾回收机制
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file active">
									<a href="/luffy/2021/08/20/04-Java%E8%99%9A%E6%8B%9F%E6%9C%BA/05.%20CMS%E5%9E%83%E5%9C%BE%E6%94%B6%E9%9B%86%E5%99%A8/">
										05. CMS垃圾收集器
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/20/04-Java%E8%99%9A%E6%8B%9F%E6%9C%BA/06.%20G1%E5%9E%83%E5%9C%BE%E6%94%B6%E9%9B%86%E5%99%A8/">
										06. G1垃圾收集器
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/24/04-Java%E8%99%9A%E6%8B%9F%E6%9C%BA/07.%20JVM%E8%B0%83%E4%BC%98/">
										07. JVM调优
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										05-Spring
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/05-Spring/01.%20SpringMVC/">
										01. SpringMVC
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/05-Spring/02.%20Spring%E5%9F%BA%E7%A1%80/">
										02. Spring基础
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/05-Spring/03.%20SpringBean%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F/">
										03. SpringBean生命周期
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										06-Redis
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/06-Redis/01.%20Redis%E5%9F%BA%E7%A1%80/">
										01. Redis基础
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/06-Redis/02.%20Redis%E6%8C%81%E4%B9%85%E5%8C%96/">
										02. Redis持久化
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/20/06-Redis/03.%20Redis%E4%B8%BB%E4%BB%8E%E6%9E%B6%E6%9E%84/">
										03. Redis主从架构
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/20/06-Redis/04.%20Redis%E5%88%86%E7%89%87%E9%9B%86%E7%BE%A4/">
										04. Redis分片集群
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										07-消息队列
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/07-%E6%B6%88%E6%81%AF%E9%98%9F%E5%88%97/01.%20Kafka%E5%9F%BA%E7%A1%80/">
										01. Kafka基础
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/07-%E6%B6%88%E6%81%AF%E9%98%9F%E5%88%97/02.%20%E4%BD%BF%E7%94%A8Kafka%E4%BC%9A%E8%80%83%E8%99%91%E4%BB%80%E4%B9%88%E9%97%AE%E9%A2%98/">
										02. 使用Kafka会考虑什么问题
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										08-MySQL
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/08-MySQL/01.%20MySQL%E7%B4%A2%E5%BC%95/">
										01. MySQL索引
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/08-MySQL/02.%20MySQL%E4%BA%8B%E5%8A%A1%E5%92%8C%E9%94%81%E6%9C%BA%E5%88%B6%E5%92%8CMVCC/">
										02. MySQL事务和锁机制和MVCC
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/19/08-MySQL/03.%20MySQL%E8%B0%83%E4%BC%98/">
										03. MySQL调优
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										09-项目场景类
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/20/09-%E9%A1%B9%E7%9B%AE%E5%9C%BA%E6%99%AF%E7%B1%BB/01.%20%E5%A6%82%E4%BD%95%E5%AE%9E%E7%8E%B0%E5%8E%BB%E9%87%8D%E5%92%8C%E5%B9%82%E7%AD%89/">
										01. 如何实现去重和幂等
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/20/09-%E9%A1%B9%E7%9B%AE%E5%9C%BA%E6%99%AF%E7%B1%BB/02.%20%E7%B3%BB%E7%BB%9F%E9%9C%80%E6%B1%82%E5%A4%9A%E5%8F%98%E5%A6%82%E4%BD%95%E8%AE%BE%E8%AE%A1/">
										02. 系统需求多变如何设计
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/20/09-%E9%A1%B9%E7%9B%AE%E5%9C%BA%E6%99%AF%E7%B1%BB/03.%20%E8%AE%BE%E8%AE%A1%E6%A8%A1%E5%BC%8F/">
										03. 设计模式
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										10-计算机网络
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/08/25/10-%E8%AE%A1%E7%AE%97%E6%9C%BA%E7%BD%91%E7%BB%9C/01.%20HTTP/">
										01. HTTP
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/10/13/10-%E8%AE%A1%E7%AE%97%E6%9C%BA%E7%BD%91%E7%BB%9C/02.TCP%E4%B8%89%E6%AC%A1%E6%8F%A1%E6%89%8B%E5%92%8C%E5%9B%9B%E6%AC%A1%E6%8C%A5%E6%89%8B/">
										02.TCP三次握手和四次挥手
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										11-算法
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/09/01/11-%E7%AE%97%E6%B3%95/01.%20%E6%8E%92%E5%BA%8F%E7%AE%97%E6%B3%95/">
										01. 排序算法
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/09/02/11-%E7%AE%97%E6%B3%95/02.%20LeetCode%20Easy/">
										02. LeetCode Easy
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/09/02/11-%E7%AE%97%E6%B3%95/02.%20LeetCode%20Medium/">
										02. LeetCode Medium
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
							<ul>
								<li class="directory">
									<a href="#" class="directory">
										<i class="fa fa-plus-square-o"></i>
										12-austin项目
									</a>
									
							<ul>
								<li class="file">
									<a href="/luffy/2021/11/01/12-austin%E9%A1%B9%E7%9B%AE/00.%E9%A1%B9%E7%9B%AE%E4%BB%8B%E7%BB%8D/">
										00.项目介绍
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/10/30/12-austin%E9%A1%B9%E7%9B%AE/01.%E7%8E%AF%E5%A2%83%E6%90%AD%E5%BB%BA/">
										01.环境搭建
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/10/30/12-austin%E9%A1%B9%E7%9B%AE/02.%E6%97%A5%E5%BF%97/">
										02.日志
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/11/24/12-austin%E9%A1%B9%E7%9B%AE/03.%E5%B7%A5%E5%85%B7%E5%8C%85/">
										03.工具包
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/11/24/12-austin%E9%A1%B9%E7%9B%AE/04.%E4%BB%8E%E4%B8%80%E6%9D%A1%E7%9F%AD%E4%BF%A1%E5%BC%80%E5%A7%8B/">
										04.从一条短信开始
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/11/24/12-austin%E9%A1%B9%E7%9B%AE/05.%E9%80%89%E6%8B%A9%E6%95%B0%E6%8D%AE%E5%BA%93%E5%92%8C%E6%8A%80%E6%9C%AF%E9%80%89%E5%9E%8B/">
										05.选择数据库和技术选型
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/11/24/12-austin%E9%A1%B9%E7%9B%AE/06.MySQL%E7%8E%AF%E5%A2%83%E6%90%AD%E5%BB%BA/">
										06.MySQL环境搭建
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
							<ul>
								<li class="file">
									<a href="/luffy/2021/11/24/12-austin%E9%A1%B9%E7%9B%AE/07.Kafka%E7%8E%AF%E5%A2%83%E6%90%AD%E5%BB%BA/">
										07.Kafka环境搭建
									</a>
								</li>
								<div class="article-toc" style="display: none;"></div>
							</ul>
			
								</li>
								
							</ul>
			
		</div>
	</div>
</div>

	<!-- 引入正文 -->
	<div id="content">
		<h1 id="article-title">

	05. CMS垃圾收集器
</h1>
<div class="article-meta">
	
	<span>3y</span>
	<span>2021-08-20 15:31:06</span>
		<div id="article-categories">
    
		<span>Categories：</span>
            
    

    
		<span>Tags：</span>
            
    
		</div>

</div>

<div id="article-content">
	<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>今天还是来聊聊CMS垃圾收集器呗？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：嗯啊…</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：如果用Seria和Parallel系列的垃圾收集器：在垃圾回收的时，用户线程都会完全停止，直至垃圾回收结束！</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gsl6dczaygj30qi0tmdip.jpg"><span style="color:#000080"><strong>候选者</strong></span>：CMS的全称：Concurrent Mark Sweep，翻译过来是「并发标记清除」</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：用CMS对比上面的垃圾收集器(Seria和Parallel和parNew)：它最大的不同点就是「并发」：在GC线程工作的时候，用户线程「不会完全停止」，用户线程在「部分场景下」与GC线程一起并发执行。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：但是，要理解的是，无论是什么垃圾收集器，Stop The World是一定无法避免的！</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：CMS只是在「部分」的GC场景下可以让GC线程与用户线程并发执行</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：CMS的设计目标是为了避免「老年代 GC」出现「长时间」的卡顿（Stop The World）</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtvor6oo8dj60re06gwfk02.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>那你清楚CMS的工作流程吗？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：只了解一点点，不能多了。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：CMS可以简单分为5个步骤：初始标记、并发标记、并发预清理、重新标记以及并发清除</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：从步骤就不难看出，CMS主要是实现了「标记清除」垃圾回收算法</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯…是的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：我就从「初始标记」来开始吧</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：「初始标记」会标记GCRoots「直接关联」的对象以及「年轻代」指向「老年代」的对象</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：「初始标记」这个过程是会发生Stop The World的。但这个阶段的速度算是很快的，因为没有「向下追溯」（只标记一层）</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gsm40eftqoj31ec0oi7cv.jpg"></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：在「初始标记」完了之后，就进入了「并发标记」阶段啦</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：「并发标记」这个过程是不会停止用户线程的（不会发生 Stop The World）。这一阶段主要是从GC Roots向下「追溯」，标记所有可达的对象。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：「并发标记」在GC的角度而言，是比较耗费时间的（需要追溯）</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gsm40366ztj31f00o8k0g.jpg"></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：「并发标记」这个阶段完成之后，就到了「并发预处理」阶段啦</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：「并发预处理」这个阶段主要想干的事情：希望能减少下一个阶段「重新标记」所消耗的时间</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：因为下一个阶段「重新标记」是需要Stop The World的</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯…</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：「并发标记」这个阶段由于用户线程是没有被挂起的，所以对象是有可能发生变化的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>： 可能有些对象，从新生代晋升到了老年代。可能有些对象，直接分配到了老年代（大对象）。可能老年代或者新生代的对象引用发生了变化…</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>那这个问题，怎么解决呢？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：针对老年代的对象，其实还是可以借助类card table的存储（将老年代对象发生变化所对应的卡页标记为dirty）</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：所以「并发预处理」这个阶段会扫描可能由于「并发标记」时导致老年代发生变化的对象，会再扫描一遍标记为dirty的卡页</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯…</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：对于新生代的对象，我们还是得遍历新生代来看看在「并发标记」过程中有没有对象引用了老年代..</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：不过JVM里给我们提供了很多「参数」，有可能在这个过程中会触发一次 minor GC（触发了minor GC 是意味着就可以更少地遍历新生代的对象）</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gsm4es0t9vj31m00r0tjy.jpg"></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：「并发预处理」这个阶段阶段结束后，就到了「重新标记」阶段</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：「重新标记」阶段会Stop The World，这个过程的停顿时间其实很大程度上取决于上面「并发预处理」阶段（可以发现，这是一个追赶的过程：一边在标记存活对象，一边用户线程在执行产生垃圾）</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gsm4mglbrcj31ls0py47v.jpg"></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：最后就是「并发清除」阶段，不会Stop The World</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：一边用户线程在执行，一边GC线程在回收不可达的对象</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：这个过程，还是有可能用户线程在不断产生垃圾，但只能留到下一次GC 进行处理了，产生的这些垃圾被叫做“浮动垃圾”</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：完了以后会重置 CMS 算法相关的内部数据，为下一次 GC 循环做准备</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gsm4pj0fc6j31n40re13a.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯，CMS的回收过程，我了解了</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>听下来，其实就是把垃圾回收的过程给”细分”了，然后在某些阶段可以不停止用户线程，一边回收垃圾，一边处理请求，来减少每次垃圾回收时 Stop The World的时间</strong></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：当然啦，中间也做了很多的优化（dirty card标记、可能中途触发minor gc等等，在我理解下，这些应该都提供了CMS的相关参数配置）</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>不过，我看现在很多企业都在用G1了，那你觉得CMS有什么缺点呢？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：1.空间需要预留：CMS垃圾收集器可以一边回收垃圾，一边处理用户线程，那需要在这个过程中保证有充足的内存空间供用户使用。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：如果CMS运行过程中预留的空间不够用了，会报错（Concurrent Mode Failure），这时会启动 Serial Old垃圾收集器进行老年代的垃圾回收，会导致停顿的时间很长。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：显然啦，空间预留多少，肯定是有参数配置的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：2. 内存碎片问题：CMS本质上是实现了「标记清除算法」的收集器（从过程就可以看得出），这会意味着会产生内存碎片</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：由于碎片太多，又可能会导致内存空间不足所触发full GC，CMS一般会在触发full GC这个过程对碎片进行整理</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：整理涉及到「移动」/「标记」，那这个过程肯定会Stop The World的，如果内存足够大（意味着可能装载的对象足够多），那这个过程卡顿也是需要一定的时间的。</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯…</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtysjs3tzoj60nm076q3e02.jpg"></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：使用CMS的弊端好像就是一个死循环：</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：1. 内存碎片过多，导致空间利用率减低。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：2. 空间本身就需要预留给用户线程使用，现在碎片内存又加剧了空间的问题，导致有可能垃圾收集器降级为Serial Old，卡顿时间更长。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：3. 要处理内存碎片的问题（整理），同样会卡顿</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：不过，技术实现就是一种trade-off（权衡），不可能你把所有的事情都做得很完美</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：了解这个过程，是非常有趣的</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：那G1垃圾收集器你了解吗</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：只了解一点点，不能多了</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：不过，留到下次吧，先让你消化下，不然怕你顶不住了。</p>
<p><strong>本文总结</strong>：</p>
<ul>
<li><p><strong>CMS垃圾回收器设计目的</strong>：为了避免「老年代 GC」出现「长时间」的卡顿（Stop The World）</p>
</li>
<li><p><strong>CMS垃圾回收器回收过程</strong>：初始标记、并发标记、并发预处理、重新标记和并发清除。初始标记以及重新标记这两个阶段会Stop The World</p>
</li>
<li><p><strong>CMS垃圾回收器的弊端</strong>：会产生内存碎片&amp;&amp;需要空间预留：停顿时间是不可预知的</p>
</li>
</ul>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gsmdclmzxyj30u01y8k8q.jpg"></p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtnap1q52rj61400mitb602.jpg"></p>
<p><span align="center">第一时间获取<span style="color:#000080"><strong>BATJTMD一线互联网大厂</strong></span>最新的面试资料以及内推机会关注公众号<span style="color:#ab4642"><strong>「对线<span style="color:#ab4642"></strong><span style="color:#ab4642"><strong>面试官</strong></span><strong></span>」</strong></span></p>
<img align="center" src='https://tva1.sinaimg.cn/large/008i3skNgy1gtlvty8zo5j60u00u0q5602.jpg' width=300px height=300px />

</div>


    <div class="post-guide">
        <div class="item left">
            
              <a href="/luffy/2021/08/20/04-Java%E8%99%9A%E6%8B%9F%E6%9C%BA/06.%20G1%E5%9E%83%E5%9C%BE%E6%94%B6%E9%9B%86%E5%99%A8/">
                  <i class="fa fa-angle-left" aria-hidden="true"></i>
                  06. G1垃圾收集器
              </a>
            
        </div>
        <div class="item right">
            
              <a href="/luffy/2021/08/19/04-Java%E8%99%9A%E6%8B%9F%E6%9C%BA/04.%20%E5%9E%83%E5%9C%BE%E5%9B%9E%E6%94%B6%E6%9C%BA%E5%88%B6/">
                04. 垃圾回收机制
                <i class="fa fa-angle-right" aria-hidden="true"></i>
              </a>
            
        </div>
    </div>




<script>
	
	
</script>
	</div>
	<button id="totop-toggle" class="toggle"><i class="fa fa-angle-double-up" aria-hidden="true"></i></button>
</body>
</html>
